1. Field of the Invention
This invention relates to a cooling structure for a multi-cylinder piston-engine cylinder block and more particularly to a coolant passage arrangement in the cooling structure.
2. Description of the Prior Art
A coolant passage arrangement in a cooling structure of a multi-cylinder piston-engine cylinder block must cool the upper sections of the engine cylinders constituting combustion chambers and walls joining two cylinders as effectively as the rest of the cylinder surface area despite the lower thermal emissivity of the former and must also uniformly cool all of the cylinders of said cylinder block.
FIGS. 1 and 2 show an example of a prior art coolant passage arrangement in a cylinder block (See page 80 of the September 1970 extra issue of Internal Combustion Engine, published by Kabushiki Kaisha Sankaidoh of Japan).
Individual cylinders in a row of cylinders 1 to 6 are separated by relatively small clearances P inside a cylinder block 7 and surrounded by a water jacket 8 defined by a continuous water jacket casing with four vertical sides 9, 10, 11 and 12. The clearance P is as small as possible in order to minimize the length of the cylinder block 7. The cylinder 2 is rigidly connected to the cylinders 1 and 3 by intervening ribs 13 and the cylinder 5 is similarly connected to the cylinders 4 and 6 by intervening ribs 13. Each of the ribs 13 includes an opening connecting the longitudinal sides of the water jacket 8 opposite the walls 9 and 10. The water jacket walls 9, 10 and 11 have bosses 18, each of which includes a smooth hole 19. Bolts pass through the holes 19 in order to clamp a cylinder head (not shown) onto the cylinder block 7.
The water jacket wall 10 has six coolant inlets 15, each of which faces directly toward the vertical axis of the corresponding cylinder 1 to 6, and has a side coolant gallery 14 extending along and to one side of the cylinders 1 to 6. The clearance C.sub.1 between cylinder 1 and upstream-side water jacket wall 11 as well as upstream ends of the water jacket walls 9 and 10 and the clearance C.sub.2 between cylinder 6 and downstream-side water jacket wall 12 as well as the downstream ends of the water jacket walls 9 and 10 are significantly larger than the clearance P.
The side coolant gallery 14 has an essentially constant cross-section throughout its length and is connected to the water jacket 8 through the coolant inlets 15. The upstream end 16 of the side coolant gallery 14 is connected to an outlet from a water pump (not shown) whereas, the downstream end 17 of the side coolant gallery 14 is closed.
With this coolant passage arrangement, the coolant leading from the water pump to the side coolant gallery 14 enters the front half of the water jacket 12 through the respective coolant inlets 15. A stream of coolant passing through each coolant inlet 15 strikes the center of the forward surface of each of the cylinders 1 to 6, i.e. the surfaces facing wall 10, and follows the forward surface to the right and left. Streams of coolant branching around the forward surfaces of the cylinders 2 to 5 pass through the openings in the ribs 13 and the clearance between the cylinders 3 and 4 to the rear half of the water jacket 8 thus cooling the opposing walls of the adjacent cylinders 2 to 5. On the other hand, streams of coolant branching around the forward surfaces of the cylinders 1 and 6 pass through the clearances between the upstream-side water jacket wall 11 and the cylinder 1, and the downstream-side water jacket wall 12 and the cylinder 6 to the rear half of the water jacket 8, thus cooling the upstream-side outer surface of the cylinder 1 and the downstream-side outer surface of the cylinder 6. Then, the coolant having cooled the cylinders 1 to 6 flows out of the cylinder block 7 to coolant passages in a cylinder head (not shown).
This prior art cooling structure entails several drawbacks. First, since the coolant passing through the side coolant gallery 14, which has an essentially constant cross-section throughout its length, enters the front half of the water jacket 8 through the coolant inlets 15 in an upstream-to-downstream order, the coolant flow around the upstream-side cylinders is greater than that around the downstream-side cylinders, which may lead to overheating of the downstream-side cylinders and concomitant abrasion of the pistons in the downstream-side cylinders. Second, the forward surfaces of the cylinders 1 to 6 exert a great resistance to coolant flow from the coolant inlets 15, thus greatly reducing the coolant flow through the openings in the rib 13, which may lead to inadequate cooling of the opposing walls of adjoining cylinders. Third, since the resistance in the clearances C.sub.1 and C.sub.2 around the cylinders 1 and 6 is smaller than in the openings through the ribs 13 and in the clearance between the cylinders 3 and 4 even though the cylinders 1 and 6, in particular the upstream-side of the cylinder 1 and the downstream-side of the cylinder 6, are more emissive than the remaining cylinders 2 to 5, the coolant flow through the clearances C.sub.1 and C.sub.2 is greater than through the openings in the ribs 13 and the clearance between the cylinders 3 and 4, which again means less cooling of the opposing walls of adjoining cylinders.